Photosynthesis

Photosynthesis underpins all life on Earth, yet this most ubiquitous process is still something of a mystery. It is amazing that the complicated biochemistry and photochemistry of photosynthesis are well known, while at the same time we do not know how CO2 diffuses from the atmosphere to chloroplasts. Two resistances dominate the pathway from the atmosphere to the sites of carboxylation. The first is diffusion of CO2 from the atmosphere to substomatal cavities via the stomata. This project considers the second, less-well-known step in the diffusion pathway, that is, from the substomatal cavities to the sites of carboxylation. This additional step is commonly described as the internal conductance. Many studies over the past two decades have shown that internal conductance (gi) is finite and imposes a limitation on photosynthesis only slightly smaller than that due to stomatal conductance (gs). Nevertheless, we still know little about what determines internal conductance, how it varies among species, or how it affects instantaneous water-use efficiency and rates of photosynthesis per unit nitrogen (PNUE).

This project is examining the implications for leaf-level photosynthesis of the resistance to CO2 movement from sub-stomatal cavities to sites of carboxylation. The effect of internal conductance on photosynthesis and water loss is being determined via a combination of measurements and modelling.

Internal conductance may affect water-use efficiency (WUE) by up to 25%. Data are modeled for Eucalyptus globulus seedlings